澳大利亚悉尼Long Reef Beach中新世古土壤岩石磁学特征及环境意义

本文对发育在澳大利亚悉尼附近的Long Reef Beach中新世古土壤剖面进行了系统的岩石磁学研究,测量了磁化率、饱和磁化强度、饱和等温剩磁、非磁滞剩磁等常温磁学参数和磁滞回线,并对所有样品进行了热磁分析.实验结果表明:全新世软土层主要磁性矿物为MD颗粒磁铁矿,磁性矿物含量与黄土高原黄土层相当.中新世老成土层随地层深度增加主要磁性矿物由磁铁矿转变为磁赤铁矿,随着磁铁矿向磁赤铁矿的转化,开始出现赤铁矿;磁性矿物粒径分布较广,以PSD颗粒为主,其次为SD颗粒,同时含有少量MD颗粒;磁性矿物含量高于黄土高原强发育古土壤层.中新世红土矿层主要磁性矿物为赤铁矿,同时含有少量磁赤铁矿和针铁矿,属于铁的富集层,赤铁矿以SD颗粒为主,含少量PSD和MD颗粒.Long Reef Beach中新世古土壤形成时期,对应着一种全球性高温多雨气候,地表化学风化作用十分强烈.丰富的降水,导致中新世老成土层发生淋溶作用,磁铁矿在向下淋溶迁移过程中逐渐氧化为磁赤铁矿和赤铁矿,铁氧化物最终在红土矿层淀积,磁赤铁矿经高温压实作用再结晶转化为赤铁矿.磁性矿物转化过程可概括为磁铁矿—磁赤铁矿化的磁铁矿—磁赤铁矿—赤铁矿,其中部分磁赤铁矿具有热稳定性,在空气(氩气)环境中加热到700℃未发生转化.

Magnetic properties of a Miocene paleosol section in Long Reef Beach,Sydney, Australia and their environmental implications

Early Miocene paleosols, dated back to 17 Ma BP, are prevalently distributed in the Sydney area, of which laterite strata was previously thought to form in tropic environments with high temperature and humidity. However, the Australia plate did not yet drift to current position during the early Miocene when Sydney paleolatitude was 45°S—50°S, thus belonging to the temperate zone. So, what were the paleoenvironments like during the laterite development remains controversial. In order to better understand and investigate the paleoclimate of Sydney during the early Miocene, a typical paleosol profile with laterite, located at the Long Reef Beach (LRB) town (northeast of Sydney), was taken for analysis, which was divided into three parts: Holocene part, Miocene Ultisol part and Miocene Laterite part.Environmental magnetism, an efficient approach to acquire environment information recorded by magnetic minerals, has been successfully used in study of loess-paleosols. Herein, this approach is first introduced to study the Miocene paleosol, i.e. LRB. Rock magnetic measurements on all samples (n=55) include low field magnetic susceptibility (χ), saturation isothermal remanent (SIRM), saturation magnetization (M_s), anhysteretic remanent magnetization (ARM), magnetic hysteresis loops, and thermomagnetic analysis (i.e. M-T & κ-T curves).Magnetic assemble in the Holocene part, which is compatible with counterparts in the Chinese loess plateau (CLP), is dominated by magnetite in the multi-domain (MD). For the Miocene Ultisol part, a trend of magnetic transformation from magnetite into maghemite against depth was observed, which coexisted with appearance of hematite. It is characteristic of higher magnetic concentration than developed paleosol in CLP and a large grain size distribution ranging from single domain (SD) to MD, and the most important is psuedo-single domain (PSD). The part of Miocene Laterite is much higher magnetic concentration and characteristic of predominant hematite with slight concentration of maghemite and goethite. The grain size of hematite is dominated by SD, at the same time showing a bit of PSD and MD.LRB Miocene paleosol is speculated to have formed in a context of exceeding rain precipitation and high temperature, where intense chemical weathering on the Earth surface occurred. The gradual alternation of magnetite into maghemite and hematite seen in the part of Miocene Ultisol can be attributed to low temperature oxidation due to eluviation caused by abundant rainfall. Maghemite, formed in the part of Miocene Ultisol, leached downwards and was transformed into stable hematite due to high temperature and recrystallization in the part of Miocene Laterite. The conversion process of magnetic minerals with increasing chemical weathering can be summarized as magnetite-magnetite core wrapped by maghemite shell-maghemite-hematite. Maghemite thermal stability was observed in a part of samples, which might be due to its coarser grain size or Fe³⁺ replacement by Al³⁺.